A cellular structure comprises tubular cells juxtaposed with regular sections, arranged along juxtaposed alignments. According to one embodiment, the cellular structure is referred to as a honeycomb structure and each cell has a hexagonal cross section. Regardless of the pattern, the cellular structure includes partitions that separate the cells and a plurality of nodes, each of which corresponds to the shared edge between several partitions. In a honeycomb cellular structure, the nodes are found at the vertexes of the hexagonal shapes and each node corresponds to the shared edge of three partitions.
Each tubular cell has two extremities that correspond to the free borders of the partitions. The first extremities of the tubular cells form a first surface of the cellular structure and the second extremities form a second surface of the cellular structure.
For certain applications, it is necessary to form a slot in at least one surface of the cellular structure to communicate certain cells with each other, in particular to obtain a drainage function in cellular structures used for acoustic treatment and arranged in an air inlet of an airplane nacelle. Thus, each partition intersecting with the slot includes a cutout that must be positioned preferably equidistant from the nodes linked by said partition.
A device for machining a slot in a surface of a cellular structure is described in document EP 1112796. Such a device includes a machining head that holds a circular saw mounted on an output shaft of a motor and two guide wheels arranged on either side of the circular saw that can pivot freely in relation to the output shaft of the motor. The guide wheels and the saw are attached along the axis of the output shaft of the motor.
The guide wheels and the circular saw are coaxial and the radius of the circular saw is greater than the radius of the guide wheels, the difference in radius being approximately equal to the depth of the slot.
To provide guidance, each guide wheel includes a series of teeth distributed evenly about the circumference of the guide wheel, said teeth being arranged in a plane perpendicular to the output shaft of the motor. The shape of each tooth fits a tubular cell. Moreover, the diameter of the guide wheel and the pitch of the teeth are determined such that the guide wheels roll without slipping on the surface of the cellular structure, the teeth penetrating the tubular cells successively. The circular saw is in an intermediate position between two series of teeth.
To form the slot, the machining head is moved above the surface of the cellular structure in a direction corresponding to the direction of the slot to be formed, the output shaft of the motor being perpendicular to said direction. When the guide wheels are rolling on the surface of the cellular structure, the teeth penetrate the cells and the guide wheels are positioned in relation to said cells by the teeth. As the circular saw cannot be moved along the axis of the output shaft of the motor in relation to the guide wheels, it is automatically correctly positioned in relation to the tubular cells.
In practice, the machining device described in document EP 1112796 makes it possible to correctly position the notch in each partition crossed by the slot if the tubular cells of the cellular structure are correctly aligned.
However, the production of cellular structures can generate alignment errors in the cells, caused for example by non-uniform expansion of the structure.
If the alignment errors are too great, the teeth of a first guide wheel cannot be inserted correctly into the cells simultaneously with the teeth of the second guide wheel, without causing a local deformation of the cellular structure. In this case, the circular saw cannot be correctly positioned equidistant at all points from the line of the cells located to the right of same (followed by the teeth of the first guide wheel on the right-hand side) and from the line of cells located to the left of same (followed by the teeth of the second guide wheel on the left-hand side). Consequently, the slot may be offset laterally and interfere with the nodes. This lateral offsetting is also accentuated by heeling of the circular saw, which also tends to increase the width of the slot.
If the slot interferes with the node, the compression strength of the cellular structure is significantly affected around this node.
To overcome this problem, the tolerances related to the alignment criteria of the cells of the cellular structure are made more stringent so that the cells arranged on either side of the slot to be formed are correctly aligned.
However, these more stringent tolerances increase the cost of the cellular structure.
Moreover, the present invention is intended to improve the device described in document EP 1112796 to enable same to machine a slot in a cellular structure with tolerances relating to the alignment criteria of the cells that are less stringent than those required by the device in the prior art, the slot formed not affecting the nodes of said structure.